As I understand mirror maps you either have the exact same start or just the same terrain and you start somewhere in it.
In both cases, assuming your empires eventually cover most of your “zone,” the important thing is using the terrain more efficiently than them. That can be better district placement, using unique districts or improvements to get a leg up (outback station, mekewap, terrace farms, ziggurats) or a unique ability that lets you use things better. The Dutch river ability, or Japan’s district ability would be examples. This is getting more yield from the same resources.
if you have the same resource base, the other option is to get outputs more efficiently. This may mean going for eureka and inspirations aggressively, or having civ abilities that boost your military units in some way- since better military units really means more

per

invested.
At the end of the day one of you has to expend enough production to beat the other, usually in Domination, so you’ve gotta use what you got better on the input side (creating yield from terrain) or output side (more efficient units, etc).
Choosing civs built around this and less around terrain gimmicks is probably wise. Although I’d really say it also matters if the map also has AI on it because that dramatically affects the value of early strategies if you can go conquer an AI instead of a human.